Ball bearing device for a swing arm

ABSTRACT

The locus of the displacement of the centers of the balls of a ball bearing is made elliptical and the radial bearing rigidity of the ball bearing in one radial direction is made greater than the radial bering rigidity in the other radial direction orthogonal to the one radial direction, or at least one of the axial length of the portion on the inner diametral surface of the inner race of the ball bearing which is fitted to a shaft and the axial length of the portion on the outer diametral surface of the outer race of the ball bearing which is fitted to a housing is made ½ or less of the bearing width of the ball bearing, whereby the radial rigidity of a ball bearing device is made greatly small as compared with the radial rigidity in the prior art and as the result, the stability of a swing arm system can be enhanced and the higher speed and higher accuracy of control become possible.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a ball bearing device for the swing armof a disc driving apparatus for a magnetic disc, an optical disc or thelike.

[0003] 2. Related Background Art

[0004] The bearing portion of a ball bearing device for a swing armaccording to the prior art, as shown in FIG. 12A of the accompanyingdrawings, has ball bearings 1 and 11 having grease enclosed therein, andthe ball bearings are used under a pre-load at predetermined positions.FIG. 12B of the accompanying drawings shows the surface pressuredistribution 14 of the ball bearing 11 in a cross-section taken alongthe line 12B-12B of FIG. 12A, and the surface pressure distribution 14is indicated by a circle, and the radial bearing rigidity of allportions in the circumferential direction thereof is constant.

[0005] As shown, there are fitting portions on both of the innerdiametral surface of the inner race of the ball bearing fitted to ashaft 5 and the outer diametral surface of the outer race fitted to ahousing 10.

[0006] These fitting portions are adhesively secured to the shaft 5 andthe housing 10 over the entire bearing width of the ball bearings 1 and11.

[0007]FIG. 12C of the accompanying drawings schematically shows theactually used state of the ball bearing device for a swing arm accordingto the prior art.

[0008] Recently, higher and higher density has been required of magneticdisc apparatuses. Therefore, the width of tracks for recording signalson the disc has become narrower and narrower, and the higher speed ofaccess to a target track and the higher accuracy of positioningperformance have been required of a swing arm carrying thereon a headfor recording and reproducing signals.

[0009] Accordingly, in order to satisfy the higher speed and higheraccuracy of control, the freedom from a torque fluctuation such as atorque spike, a low torque, etc. are required of the ball bearings 1 and11 supporting the swing arm 9. Also, the high stability and low torqueof the control of the swing arm system when controlled at a high speedare desired.

[0010] Heretofore, in the ball bearing device for the swing arm, asshown in FIG. 12A, the two ball bearings 1 and 11 have been accuratelyassembled together with a pre-load applied in the axial directionthereof to thereby achieve the higher accuracy of the bearing device.The present invention has as its object to provide a ball bearing devicefor a swing arm in which the stability of a swing arm system is enhancedand the higher speed and higher accuracy of control are possible.

SUMMARY OF THE INVENTION

[0011] In the present invention, in order to enhance the stability ofthe swing arm and the swing arm system and achieve the higher speed andhigher accuracy of the control of the swing arm, attention has been paidto the rigidity of the ball bearing device in the radial directionthereof and it has been confirmed that it is effective to vary therigidity in the radial direction in accordance with the followingmethods.

[0012] 1) During the operation of the ball bearing device, the locus ofthe displacement of the centers of the balls is made elliptical, wherebythe rigidity of the ball bearing device in the radial direction thereofis made to differ between two directions orthogonal to each other in theradial direction.

[0013] Particularly in the case of a straight swing arm, it ispreferable that the rigidity in a radial direction parallel to the axisof the arm be made great relative to the rigidity in a radial directionorthogonal to the direction, and further it is preferable in enhancingthe stability of the control of the swing arm to make the magnitudethereof greater by 5% or more.

[0014] 2) The rigidity of the ball bearing device in the radialdirection thereof is greatly reduced as compared with the rigidity ofthe ball bearing device of the prior-art construction in the radialdirection thereof.

[0015] It is particularly preferable in enhancing the stability of thecontrol of the swing arm that the rigidity in the radial direction bereduce to 50% or less relative to the rigidity of the ball bearingdevice of the prior-art construction in the radial direction thereof.

[0016] According to one aspect of the present application, the ballbearing of the present invention has a difference between the radialbearing rigidities of the ball bearing in two directions orthogonal toeach other.

[0017] In the present invention, the locus of the displacement of thecenters of the balls of the ball bearing is made elliptical, whereby theamount of elastic deformation of the balls and raceway surface is madeto differ between the major axis side and the minor axis side of theellipse. As the result, the contact surface pressure differs between themajor axis direction and the minor axis direction of the ellipse and theradial bearing rigidity differs. Or further, if the radial bearing gapin the other radial direction is made larger than the radial bearing gapin one radial direction orthogonal to the other radial direction, theballs and the raceway surface do not contact with each other in theother radial direction, and the radial bearing rigidity in the otherradial direction can be made small as compared with the radial bearingrigidity in one radial direction. As the result, the radial bearingrigidities of the ball bearing in the two directions orthogonal to eachother can be made to have a difference therebetween.

[0018] Specifically, in the ball bearing according to the presentinvention, it is preferable that balls be disposed between one racewaysurface provided on an inner member and the other raceway surfaceprovided on an outer member and at least one of one raceway surface andthe other raceway surface be elliptical in its cross-section by a radialplane. Or further, it is preferable that the radial bearing gap in oneradial direction be smaller than the other radial bearing gap orthogonalto the one radial direction. For example, in the ball bearing accordingto the present invention, the inner member may fit to a shaft and in oneradial direction, the inner member may be pressed into the shaft, and inthe other radial direction, a gap may be provided between the innermember and the shaft. Or in the ball bearing according to the presentinvention, the outer member may fit to a housing and in one radialdirection, the outer member may be pressed into the housing, and in theother radial direction, a gap may be provided between the outer memberand the housing.

[0019] Or further, in the ball bearing according to the presentinvention, the inner member may fit to the shaft and the outer membermay fit to the housing, and a plane containing the centers of aplurality of balls disposed between one raceway surface of the innermember and the other raceway surface of the outer member may be inclinedwith respect to a plane perpendicular to the center axis of the rockingmember of one of the shaft and the housing.

[0020] According to one aspect of the present application, the ballbearing according to the present invention is characterized in that atleast one of the axial length of the portion fitting to the shaft in theinner diametral surface of an inner race and the axial length of theportion fitting to the housing in the outer diametral surface of anouter race is ½ or less of the bearing width of the ball bearing.Thereby, the rigidity of the ball bearing device in the radial directionthereof is greatly reduced as the rigidity of the ball bearing device ofthe prior-art construction in the radial direction thereof.

[0021] According to one aspect of the present application, the ballbearing device of the present invention is characterized in that theradial bearing rigidities in two directions orthogonal to each otherhave a difference therebetween and at least one of the axial length ofthe portion fitted to the shaft in the inner diametral surface of theinner race and the axial length of the portion fitted to the shaft inthe outer diametral surface of the outer race is ½ or less of thebearing width of the ball bearing.

[0022] Other modes of the present invention will become apparent fromthe embodiments of the invention which will be shown and describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1A is a cross-sectional view showing a ball bearing deviceaccording to a first embodiment of the present invention.

[0024]FIG. 1B shows the surface pressure distribution of the portion ofcontact between a raceway surface and balls in the first embodiment ofthe present invention.

[0025]FIG. 2 is a cross-sectional view showing a ball bearing deviceaccording to a second embodiment of the present invention.

[0026]FIGS. 3A, 3B and 3C are cross-sectional views showing a ballbearing device according to a third embodiment of the present invention.

[0027]FIG. 4A is a cross-sectional view showing a ball bearing deviceaccording to a fourth embodiment of the present invention.

[0028]FIG. 4B shows the surface pressure distribution of the portion ofcontact between a raceway surface and balls in the fourth embodiment ofthe present invention.

[0029]FIG. 5A is a cross-sectional view showing a ball bearing deviceaccording to a fifth embodiment of the present invention.

[0030]FIG. 5B shows the surface pressure distribution of the portion ofcontact between a raceway surface and balls in the fifth embodiment ofthe present invention.

[0031]FIG. 6A is a cross-sectional view showing a ball bearing deviceaccording to a sixth embodiment of the present invention.

[0032]FIG. 6B shows the surface pressure distribution of the portion ofcontact between a raceway surface and balls in the sixth embodiment ofthe present invention.

[0033]FIG. 7A is a schematic cross-sectional view of a ball bearingdevice according to a seventh embodiment of the present invention.

[0034]FIG. 7B is a schematic longitudinal cross-sectional view of theball bearing device according to the seventh embodiment of the presentinvention when the device is used in an outer race turning type actuatorhaving a voice coil motor.

[0035]FIG. 8 is a schematic longitudinal cross-sectional view of a ballbearing device according to an eighth embodiment of the presentinvention in which the rigidity of a housing is reduced.

[0036]FIG. 9 is a schematic cross-sectional view of a ball bearingdevice according to a ninth embodiment of the present invention.

[0037]FIG. 10 is a schematic longitudinal cross-sectional view of a ballbearing device according to a tenth embodiment of the present invention.

[0038]FIG. 11A is a schematic longitudinal cross-sectional view of aball bearing device according to an eleventh embodiment of the presentinvention.

[0039]FIG. 11B is a cross-sectional view of the ball bearing device ofFIG. 11A taken along the line 11B-11B of FIG. 11A.

[0040]FIG. 12A is a cross-sectional view showing a ball bearing deviceaccording to the prior art.

[0041]FIG. 12B shows the surface pressure distribution of the portion ofcontact between a raceway and balls in the prior art.

[0042]FIG. 12C is a schematic longitudinal cross-sectional view of theball bearing device according to the prior art when the device is usedin an outer race turning type actuator having a voice coil motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] A first embodiment of the present invention will hereinafter bedescribed with reference to the drawings. Like constituents in variousembodiments are designated by like reference numerals.

[0044] Means for making the locus of the displacement of the center of aball elliptical and making the rigidity of a ball bearing in the radialdirection thereof differ between two radial directions orthogonal toeach other includes the following means:

[0045] 1. To work the raceway surface of at least one of an inner raceand an outer race into an elliptical shape in advance.

[0046] 2. During the incorporation of the ball bearing, to incline aplane containing the centers of a plurality of balls with respect to aplane perpendicular to the center axis of the rocking member of at leastone of a shaft and a housing.

[0047] 3. In the other radial direction, there is a gap between the ballbearing and the housing or the shaft, and in one radial directionorthogonal to the other direction, to deform the raceway surface of oneof the outer race and the inner race into an elliptical shape by theeffect of the gap when the ball bearing is pressed into the housing orthe shaft.

[0048] 4. To apply additional working by which thermal deformation iscreated in the outer diametral surface or the inner diametral surface ofthe ball bearing, and deform the raceway surface of one of the outerrace and the inner race.

[0049] Description will hereinafter be made in accordance with thedrawings.

[0050]FIG. 1A shows a first embodiment of shaft fixing using a ballbearing according to a first aspect of the present invention. In a deepgroove ball bearing 1 (hereinafter referred to as the ball bearing), aplurality of balls 15 retained by a retainer are disposed between oneraceway surface provided on the inner races 3, 13 of an inner member(hereinafter referred to as one raceway surface) and the other racewaysurface provided on the outer races 2, 12 of an outer member(hereinafter referred to as the other raceway surface). One racewaysurface is such that its cross-section by a radial plane is circular,and the other raceway surface is such that its cross-section by a radialplane is elliptical. As the result, the ball bearings 1, 11 are suchthat the radial bearing gap in one radical direction is smaller than theother radial bearing gap orthogonal to the one radial direction.

[0051] The ball bearings 1, 11 have their outer diametral surfacesfixedly fitted to the inner diametral surface of the cylindrical housing10 of a magnetic disc apparatus at an axial interval and have theirinner diametral surfaces fixedly fitted to a shaft 5 fixed to a base 16.The minor axis of the ellipse of the other raceway surface provided onthe outer races 2, 12 is incorporated so as to become parallel to theaxial direction of a swing arm 9 fixedly fitted to the outer diametralsurface of the housing 10. The shapes and direction of the minor axis ofthe ellipses of the raceway surfaces are made coincident with each otherin the ball bearings 1 and 11. When with the shaft thus fixed, thehousing 10 is used as a rocking member, if the raceway surfaces of theouter races 2, 12 on that side of the magnetic disc apparatus which ismounted on the housing 10 are worked into an elliptical shape, the axialdirection of the swing arm 9 and the minor axis of the ellipse of theraceway surface will always become parallel to each other even if theswing arm 9 effects rocking movement.

[0052] When the raceway surfaces are thus worked into an ellipticalshape in advance, the raceway surfaces can be made into an ellipticalshape even if the ball bearings 1, 11 are used while being looselyfitted to the housing 10 and the shaft 5.

[0053]FIG. 1B shows a cross-section taken along the line 1B-1B of FIG.1A and the surface pressure distribution 14 of the portion of contactbetween the balls 15 and raceway surface of the ball bearing used in thecross-section. The dimensions of the major axis and the minor axis areset such that the surface pressure of the ellipses of the racewaysurfaces of the outer races in the direction of the minor axis thereofis about twice as great as the surface pressure in the direction of themajor axis. As described above, the ball bearing according to the firstembodiment of the present invention is such that the locus of thedisplacement of the centers of the balls 15 is elliptical, and as theresult, the radial bearing rigidity in one radial direction, i.e., thedirection of the minor axis, is greater than the radial bearing rigidityin the other radial direction orthogonal to the one radial direction,i.e., the direction of the major axis.

[0054]FIG. 2 shows a second embodiment of the present invention in whichthe shaft 5 is a rocking member. The ball bearings 1, 11 in which inadvance, the raceway surface of one of the inner members, i.e., theinner races 3, 13, is worked so that its cross-section by a radial planemay become elliptical and the raceway surface of the other of the outerraces 2, 12 is worked so that its cross-section by a radial plane maybecome circular are incorporated in the housing 10 fixed to the base 16of the magnetic disc apparatus, in a direction in which the major axisof the ellipses of the raceway surfaces of the inner races 3, 13 isparallel to the axial direction of the swing arm 9 fixed to the shaft 5.The shapes and directions of the major axes of the ellipses of theraceway surfaces are made coincident with each other by the two upperand lower ball bearings 1 and 11. As the result, the ball bearings aresuch that the radial bearing gap in one radial direction is smaller thanthe other radial bearing gap orthogonal to the one radial direction.

[0055] When the raceway surfaces are thus worked into an ellipticalshape in advance, the raceway surfaces can be made elliptical even ifthe ball bearings 1, 11 are used while being loosely fitted to thehousing 10 and the shaft 5. As described above, the ball bearingaccording to the second embodiment of the present invention is such thatthe locus of the displacement of the centers of the balls 15 iselliptical and as the result, the radial bearing rigidity in one radialdirection is greater than the radial bearing rigidity in the otherradial direction orthogonal to the one radial direction.

[0056]FIGS. 3A to 3C show a third embodiment of the present invention.The swing arm in this embodiment is such that the ball bearings 1, 11 inwhich the raceway surface of one of the inner races 3, 13 and theraceway surface of the other of the outer races 2, 12 are worked into anordinary standard are assembled to the shaft 5 and the housing 10 in astate in which a plane containing the centers of a plurality of balls 15is inclined with respect to a plane perpendicular to the center axis ofthe rocking member of one of the shaft 5 and the housing 10, whereby thelocus of the displacement of the centers of the balls 15 is madeelliptical. That is, as in the ordinary standard ball bearing, oneraceway surface and the other raceway surface are circular in theircross-section by a radial plane. In the present embodiment, by thestandard ball bearings 1, 11 being inclined and mounted, the balls 15are displaced in the axial bearing gaps of the ball bearings 1, 11 inthe axial direction of the ball bearings 1, 11, whereby the locus of thedisplacement of the centers of the balls 15 is made elliptical. Theinner diametral surfaces of the inner races 3, 13 are parallel to thecenter axis of the shaft 5, and the outer diametral surfaces of theouter races 2, 12 are perpendicular to a plane containing the centers ofthe plurality of balls 15.

[0057]FIG. 3A shows a case where the upper and lower ball bearings 1 and11 have been symmetrically inclined and mounted and thereafter aconstant position pre-load has been applied thereto. The center axis ofthe shaft 5 is concentric with the center axis of the housing 10.

[0058]FIG. 3B shows a case where the upper and lower ball bearings 1 and11 have been inclined in the same direction and mounted and thereafter apre-load has been applied thereto. The center axis of the shaft 5 isconcentric with the center axis of the housing 10.

[0059]FIG. 3C shows a case where the upper and lower ball bearings 1 and11 have been inclined in the same direction and mounted and thereafter aconstant position pre-load has been applied thereto. The center axis ofthe shaft 5 is inclined with respect to the center axis of the housing10.

[0060] In any of the above-described three examples, the planecontaining the centers of the plurality of balls become inclined withrespect to the plane perpendicular to the center axis of the rockingmember of one of the shaft 5 and the housing 10, and as the result, thetack locus becomes elliptical. As described above, in the presentembodiment, the radial bearing rigidity of the ball bearings in oneradial direction becomes greater than the radial bearing rigidity in theother radial direction orthogonal to the one radial direction, and theradial bearing gap in one radial direction becomes smaller than theradial bearing gap in the other radial direction orthogonal to the oneradial direction. As the result, again in the present embodiment, thereis obtained an effect similar to that of the first embodiment.

[0061] In the case of this third embodiment, at least one of theordinary ball bearings 1, 11, the shaft 5 and the housing 10 can beslightly worked and used and therefore, the cost is low.

[0062]FIGS. 4A and 4B show a fourth embodiment of the present invention.FIG. 4B shows a cross-section taken along the line 4B-4B of FIG. 4A andthe surface pressure distribution in the cross-section, and in thisembodiment, two ordinary ball bearings are pressed into the housing 10to thereby make the outer diametral surfaces and raceway surfaces of theouter races elliptical. On the inner diametral surface of the housing10, escapes 25 are formed in advance on the opposite sides of the centeraxis of the ball bearings 1, 11 in one radial direction. When this isdone, the portion of the escapes 25 in the ball bearings 1, 11 becomesloosely fitted and the pressed-in portion in the other radial directionorthogonal to one radial direction linking the escapes 25 togetherbecomes tightly fitted, and a compression load acts on the outer races2, 12 of the ball bearings 1, 11 in the pressed-in portion. As theresult, the outer diametral surfaces and raceway surfaces of the outerraces 2, 12 are deformed into an elliptical shape in which thecross-section by the radial plane has the portion of the escapes 25 asthe major axis. If these escapes 25 are provided in a directionorthogonal to the axial direction of the swing arm 9 mounted on theshaft 5, the radial bearing rigidity in a direction parallel to theaxial direction of the swing arm can be made greater than in a directionorthogonal to the axial direction of the swing arm 9. The racewaysurface of one of the inner races 3, 13 is circular in its cross-sectionby the radial plane. The effect thereafter is substantially similar tothat of the aforedescribed embodiments. This embodiment enables ordinaryball bearings to be used and therefore is low in cost. Also, the housing10 can only be provided with the escapes 25 and therefore, the factorsfor an increased cost are few in the working of the housing as well.

[0063]FIGS. 5A and 5B show a fifth embodiment of the present invention.FIG. 5B shows a cross-section taken along the line 5B-5B of FIG. 5A anda surface pressure distribution in the cross-section, and in thisembodiment, as in the fourth embodiment, the ball bearings 1, 11 arepressed into the housing 10 to thereby make the raceway surfaces of theouter races 2, 12 elliptical. The difference of the present embodimentfrom the fourth embodiment is that the escapes 25 are provided not onthe housing 10 but on the outer diametral surfaces of the outer races 2,12.

[0064]FIGS. 6A and 6B show a sixth embodiment of the present invention.FIG. 6B shows a cross-section taken along the line 6B-6B of FIG. 6A anda surface pressure distribution in the cross-section, and in thisembodiment, an energy beam such as a laser beam is locally applied tothe outer races 2, 12 of ordinary ball bearings 1, 11 to thereby causethermal deformation, thus deforming (thermally deformed portions 26) theouter diametral surfaces and the other raceway surface of the outerraces 2, 12 so that the cross-section by a radial plane may becomeelliptical. In the present embodiment, the laser is applied to twoportions of the outer diametral surface of the outer races 2, 12 whichare spaced apart by 180° from each other to thereby deform the outerdiametral surfaces and raceway surfaces of the outer races 2, 12 into anellipse, whereafter the ball bearings 1, 11 are mounted on the housing10 as by loose fitting. One raceway surface of the inner races 3, 13 iscircular in its cross-section by a radial plane. In the other points,the effect of the present embodiment is substantially similar to that ofthe embodiments hitherto described.

[0065] Both of one raceway surface and the other raceway surface may beelliptical in their cross-sections by a radial plane, and the major axisof the ellipse of one raceway surface and the major axis of the ellipseof the other raceway surface may be orthogonal to each other.

[0066] Also, even if in one radial direction, the inner member ispressed into the shaft 5 and in the other radial direction orthogonal tothe one radial direction, there is a gap between the inner member andthe shaft 5, one raceway surface of the inner races 3, 13 of the innermember becomes elliptical in its cross-section by a radial plane. Inthis case, the other raceway surface of the outer member may be circularor elliptical in its cross-section by a radial plane.

[0067] As regards the ball bearings 1, 11, if the radial bearingrigidity in one radial direction is greater than the radial bearingrigidity in the other radial direction orthogonal to the one radialdirection, the radial bearing gap becomes larger in the other radialdirection than in the one radial direction and a low torque is provided.Accordingly, there can be provided a ball bearing in which the radialbearing rigidity in the necessary radial direction is made great andwhich is of a low torque as a whole.

[0068] Also, other embodiment in which the raceway surfaces of the ballbearings are elliptical than the above-described embodiments will do,and an embodiment in which the locus of the displacement of the centersof the balls is elliptical will also do, and any of these embodimentscan provide an effect similar to that of the present invention.

[0069] As regards the ball bearings 1, 11, with the inner member as ashaft, one raceway surface may be provided on the shaft, and with theouter member as a housing, the other raceway surface may be provided onthe housing.

[0070] A second aspect of the present invention will now be described.

[0071]FIG. 7A shows a seventh embodiment of a ball bearing deviceaccording to a second aspect of the present invention. The ball bearings1, 11 have a plurality of balls retained by a retainer between outerraces 2, 12 and inner races 3, 13.

[0072] These ball bearings 1, 11 are incorporated in the axiallyopposite end portions of the inner diametral surface of a housing 10 onwhich the swing arm 9 of a magnetic disc apparatus is mounted. The upperand lower ball bearings 1 and 11 are fixed to the housing 10 by lightpress-in fit on the outer diametral surfaces 2 a and 12 a of therespective outer races 2 and 12. Both of the two outer races 2, 12 havetheir axially inner sides in contact with a stepped surfaceperpendicular to the axis of the housing 10.

[0073] The ball bearings 1, 11 have the axial lengths 2 l and 12 l oftheir portions fitted to the housing 10 on the outer diametral surfaces2 a and 12 a of the outer races 2 and 12 set to ½ or less of the bearingwidths 1 l and 11l of the ball bearings 1 and 11. The inner race 13 ofthe lower ball bearing 11 has a shaft 5 fixed to its inner diametralsurface by light press-in. The portions of the two ball bearings 1; 11which are fitted to the housing 10 on the outer diametral surfaces 2 aand 12 a of the outer races are axially inner portions, i.e., the insideportions of the housing 10.

[0074] Also, as regards the shaft 5, a flange portion 6 provided at theaxially central portion is larger in diameter than the axially oppositeportions of the flange portion 6 and further, for the ease of assemblyand the enlargement of a pair of bearing spans, the inner race 13 of thelower ball bearing has its axially inner side in contact with thestepped surface of the flange portion 6. The inner diametral surface 3 aof the inner race 3 of the upper ball bearing 1 is fixed to the shaft 5by light press-in while pre-load setting by a resonance frequency iseffected. Furthermore, for the prevention of the slippage by a shockload, an adhesive agent is applied to the upper side 13 b of the innerrace 13 of the lower ball bearing, which is thus fixed to the shaft 5.

[0075] For the reduction of the radial rigidity and the ease of washing,the shaft 5 is a hollow shaft having a hole axially extending throughthe central portion of the shaft 5 and further, threaded grooves areformed from the upper and lower opening end portions of the hole so thatmounting screws can be mounted in the shaft 5. The material of the shaftmay be stainless steel, but it is preferable to use a material having asmall modulus of longitudinal elasticity such as aluminum in order toreduce the radial rigidity.

[0076] Further, as in an eighth embodiment shown in FIG. 8, slits 10 acan be formed in the outer diametral surface of the housing 10 in theportions between the two ball bearings 1 and 11 which are near the twoball bearings to thereby reduce the radial rigidity of the housing 10.In this case, the housing 10 has the outer diameter of its portionslocated on the ball bearing 1, 11 sides axially outward of thoseportions formed with the slits 10 a made smaller than the outer diameterof the portion between the upper and lower slits 10 a at the center ofthe housing 10 and a level difference A is provided on the housing 10.

[0077] Accordingly, the housing 10 is elastically deformable without itsportions axially outward of the slits 10 a contacting with a swing arm 9which will be described later. An escape portion may be provided on theswing arm 9 mounted on the housing 10 so that the portions of thehousing 10 which are axially outward of the slits 10 a may not contactwith the swing arm 9.

[0078] As the ball bearings 1, 11, use may be made of a combination ofballs of bearing steel and grease lubrication, but preferably, thematerial of the balls may be ceramics and oil may be used as a lubricantand further, a slight quantity of oil may preferably be poured into theraceway surfaces of the outer races 2, 12 and the inner races 3, 13 inadvance. Alternatively, use may be made of the so-called oil platingwhich is a lubricating method of thinly applying a slight quantity ofoil to the raceway surfaces of the outer races 2, 12 and the inner races3, 13 in advance.

[0079] By using the ball bearings 1, 11 having their raceway surfacessubjected to the oil plating, the initial scattering of the lubricantfrom the ball bearings 1, 11 can be reduced and torque fluctuation canalso be reduced as compared with the case of grease lubrication. Also,since the material of the balls is ceramics, there can be provided ballbearings 1, 11 which are low in wear and excellent in frettingdurability.

[0080] It is preferable to design the ball bearings 1, 11 so as to bereduced in radial rigidity by choosing at least one of groups comprisingmaking the radial gap greater than 15 μm exceeding the standard of 6 to14 μm, or reducing the number of balls to six or less which is smallerthan the standard of 7 to 8, and making the pre-load applied to the ballbearings 1, 11 less than 0.5 Kgf smaller than the standard of 0.5 to 1.0Kgf. As the ball bearings 1, 11 used in the present invention, use canbe made of standard ball bearings and therefore, the ball bearing devicefor a swing arm can be made inexpensively.

[0081] While the material of the retainer of the ball bearings 1, 11 maybe stainless steel, iron, nylon or like material, it is more preferableto use a polymer member containing lubricating oil formed with syntheticresin containing lubricating oil. When a retainer comprising the polymermember containing lubricating oil is used, torque and torque fluctuationbecome small and further, the lubricating oil contained in the retaineroozes for a long time and therefore, good lubrication is effected for along time and excellent durability is obtained.

[0082]FIG. 7B shows the seventh embodiment as it is actuallyincorporated into an apparatus. In this embodiment, the upper and lowerend portions of the shaft 5 are fixed to a body case 8 by means of upperand lower bolts 7. When it is desired to further reduce the radialrigidity of the shaft 5, there may be adopted a cantilever structure inwhich only the lower portion of the shaft 5 is fixed to the body case 8and the upper portion of the shaft 5 is not supported.

[0083] While the housing 10 is fitted and adhesively secured to theinner diametral surface of the swing arm 9, it may be fixed to the swingarm 9 by means of a bolt. When the housing 10 is to be adhesivelysecured, a circumferential groove-like adhesive reservoir 20 provided inthe outer diametral surface of the housing 10 is filled with an adhesiveagent.

[0084]FIG. 9 shows a ninth embodiment of the present invention. In thisembodiment, the axial lengths 3 l and 13 l of the portions of the innerdiametral surfaces of the inner races 3 and 13 of the upper and lowerball bearings 1 and 11 which are fitted to the shaft 5 are ½ or less ofthe bearing widths 1 l and 11 l of the ball bearings 1 and 11. Unlikethe seventh embodiment, the width of the housing 10 can be made the sameas the width in the prior art and the housing 10 can secure the samemounting width as the mounting width for the swing arm 9 in the priorart.

[0085]FIG. 10 shows a tenth embodiment of the present invention. In thisembodiment, the axial lengths 2 l and 12 l of the portions of the outerdiametral surfaces of the outer races 2, 12 of the ball bearings 1, 11which are fitted to the housing 10 are ½ or less of the bearing widths 1l and 11 l of the ball bearings 1, 11. The difference of the presentembodiment from the seventh embodiment is that the axially insideportions of the outer diametral surfaces 2 a and 12 a of the outer racesare not in contact with the housing 10.

[0086] That is, in both of the two ball bearings 1 and 11, the portionsof the outer diametral surfaces of the outer races 2 and 12 which arefitted to the housing 10 are axially outside portions. Therefore, theaxial length of the housing 10 is enlarged and the moment rigiditythereof is improved.

[0087] Also, in the present embodiment, as in the ninth embodiment, thehousing 10 can secure the same mounting width as the mounting with forthe swing arm 9 in the prior art.

[0088] The ball bearings 1, 11 may be such that both of the axial length3 l, 13 l of the portions of the inner diametral surfaces 3 a, 13 a ofthe inner races which are fitted to the shaft 5 and the axial lengths 2l, 12 l of the portions of the outer diametral surfaces 2 a, 12 a of theouter races which are fitted to the housing 10 are ½ or less of thebearing widths 1 l, 11 l of the ball bearings.

[0089] Also, in the present invention, the radial rigidity of the ballbearing device may be reduced by the combination of the ball bearings 1,11 are the housing 10, or radial rigidity of the ball bearing device maybe reduced by the combination of the ball bearings 1, 11 and the shaft5, or the radial rigidity of the ball bearing device may be reduced bythe combination of the ball bearings 1, 11, the housing 10 and the shaft5.

[0090] A third aspect of the present invention will now be described.

[0091]FIGS. 11A and 11B show an eleventh embodiment of a ball bearingdevice according to the third aspect of the present invention.

[0092]FIG. 11B is a cross-sectional view taken along the line 11B-11B ofFIG. 11A, and in this embodiment, two ordinary ball bearings 1 and 11are pressed into the housing 10 to thereby make the outer diametralsurfaces and raceway surfaces of the outer races 2, 12 elliptical. Onthe inner diametral surface of the housing 10, escapes 25 are formed inadvance on the opposite sides of the center axis of the ball bearings 1,11 in one radial direction. If this is done, the ball bearings 1, 11 areloosely fitted in the portions of the escapes 25 and are tightly fittedin the pressed-in portion in the other radial direction orthogonal tothe one radial direction, and a compression load acts on the outer races2, 12 of the ball bearings in the pressed-in portion.

[0093] As the result, the outer diametral surfaces and racewaysurfaces-of the outer races 2, 12 have their cross-sections by a radialplane deformed into an elliptical shape in which the portions of theescapes 25 are the major axis. If these escapes 25 are provided in adirection orthogonal to the axial direction of a swing arm 9 mounted onthe housing 10, the radial bearing rigidity in a direction parallel tothe axial direction of the swing arm can be made greater than in adirection orthogonal to the axial direction of the swing arm. Theraceway surfaces of the inner races 3, 13 are circular in theircross-sections by a radial plane.

[0094] The above-described construction is similar to the constructionof the fourth embodiment, and the other portions of the illustratedembodiment are constructed substantially similarly to the seventhembodiment.

[0095] The ball bearings 1, 11 and the ball bearing device in thepresent embodiment not only have their rigidity in the radial directiongreatly reduced as compared with the rigidity in the radial direction inthe prior art, but also the rigidities of the ball bearings 1, 11 in theradial direction are made to differ between two directions orthogonal toeach other and as the result, the stability of the swing arm system canbe enhanced and the higher speed and higher accuracy of control arepossible.

What is claimed is:
 1. A ball bearing device for a swing arm wherein aball bearing is such that radial bearing rigidity in one radialdirection is greater than radial bearing rigidity in a directionorthogonal to said one radial direction.
 2. A ball bearing device for aswing arm according to claim 1, wherein when the swing arm has a linearshape, the ball bearing is such that radial bearing rigidity in a radialdirection parallel to the axial direction of the swing arm is greaterthan radial bearing rigidity in a direction orthogonal to the axialdirection of the swing arm.
 3. A ball bearing device for a swing armaccording to claim 1, wherein in a ball bearing used in said ballbearing device for a swing arm, at least one of a raceway surface formedon the inner diametral surface of an inner race and a raceway formed onan outer race has its cross-section in a radial direction formed inadvance so as to be elliptical.
 4. A ball bearing device for a swing armcomprising: a ball bearing including balls, an inner race, an outer raceand a retainer; a shaft fitted to an inner diametral surface of saidinner race of said ball bearing; and a housing fitted to an outerdiametral surface of said outer race of said ball bearing, wherein atleast one of (1) a portion between the shaft and the inner diametralsurface of said inner race and (2) a portion between the housing and theouter diametral surface of said outer race is formed with a radialbearing gap.
 5. A ball bearing device according to claim 4, whereinsecond radial bearing gap is formed at a location opposed to said radialbearing gap in a radial direction.
 6. A ball bearing device for a swingarm comprising: a ball bearing including balls, an inner race, an outerrace and a retainer; a shaft fitted to an inner diametral surface ofsaid inner race of said ball bearing; and a housing fitted to an outerdiametral surface of said outer race of said ball bearing, wherein aplane containing the centers of said balls being inclined with respectto a plane perpendicular to the center axis of one of a rocking shaftand the housing.
 7. A ball bearing device for a swing arm comprising: aball bearing including balls, an inner race, an outer race and aretainer; a shaft fitted to the inner diametral surface of said innerrace of said ball bearing; and a housing fitted to the outer diametralsurface of said outer race of said ball bearing, wherein at least one of(1) the axial length of the portion on the inner diametral surface ofsaid inner race which is fitted to the shaft and (2) the axial length ofthe portion on the outer diametral surface of said outer race which isfitted to the housing is ½ or less of the bearing width of said ballbearing.
 8. A ball bearing device according to claim 7, wherein saidhousing is formed with a recess.
 9. A ball bearing device according toclaim 7, wherein said ball bearing is such that radial bearing rigidityin one radial direction is greater than radial bearing rigidity in adirection orthogonal to said one radial direction.
 10. A method ofstabilizing the high-speed control of a ball bearing device for a swingarm, wherein at least one of a raceway surface formed on an innerdiametral surface of an inner race of a ball bearing and a racewaysurface formed on an outer race of the ball bearing is formed in advanceso that a cross-section thereof in a radial direction may be elliptical,thereby making a radial bearing rigidity of the ball bearing of saidball bearing device in one radial direction thereof greater than theradial bearing rigidity thereof in a direction orthogonal to said oneradial direction.
 11. A method of stabilizing the high-speed control ofa ball bearing device for a swing arm, wherein at least one of (1) aportion between a shaft and an inner diametral surface of an inner raceof a ball bearing and (2) a portion between a housing and an outerdiametral surface of an outer race of the ball bearing is formed with aradial bearing gap, thereby making the radial bearing rigidity of theball bearing of said ball bearing device in one radial direction thereofgreater than the radial bearing rigidity in a direction orthogonal tosaid one radial direction.
 12. A method according to claim 11, whereinsecond radial bearing gap is formed at a location opposed to said radialbearing gap in the radial direction, thereby making the radial bearingrigidity of the ball bearing of said ball bearing device in one radialdirection thereof greater than the radial bearing rigidity in adirection orthogonal to said one radial direction.
 13. A method ofstabilizing the high-speed control of a ball bearing device for a swingarm, wherein a plane containing the centers of the balls of the ballbearing device is inclined with respect to a plane perpendicular to thecenter axis of one of a rocking shaft and a housing, thereby making theradial bearing rigidity of the ball bearing of said ball bearing devicein one radial direction thereof greater than the radial bearing rigiditythereof in a direction orthogonal to said one radial direction.
 14. Amethod of stabilizing the high-speed control of a ball bearing devicefor a swing arm, wherein at least one of the axial length of the portionof a shaft which is fitted to the inner diametral surface of the innerrace of a ball bearing and the axial length of the portion of a housingwhich is fitted to the outer diametral surface of the outer race of theball bearing is made ½ or less of the bearing width of said ballbearing, thereby reducing the radial bearing rigidity of the ballbearing device.
 15. A method according to claim 14, wherein said housingis formed with a recess, thereby reducing the radial bearing rigidity ofthe ball bearing device.
 16. A method according to claim 14, wherein theradial bearing rigidity of the ball bearing in one radial directionthereof is made greater than the radial bearing rigidity thereof in adirection orthogonal to said one radial direction.